Primary batteries such as an alkali dry battery and a nickel dry battery and secondary batteries such as a lithium ion secondary battery are used in various electronic apparatuses. In particular, the lithium ion secondary battery (hereinafter simply referred to as “lithium ion battery”) is used as a power source for a portable information terminal apparatus such as a cellular phone. The lithium ion battery is light in weight and can be used for a long period because a relatively large amount of energy can be stored therein Further, the lithium ion battery has high energy density, has excellent self-discharge properties (energy retention properties), and has substantially no problem of inter-electrode short-circuit due to deposition of dendrite substances. Therefore, the lithium ion battery can be used as a power supply not only for the terminal apparatus but also for a hybrid automobile and the like. It is also expected that the lithium ion battery will be mounted on an electric automobile now under development.
In general, various batteries including the lithium ion battery have high sealing properties. In some cases, an internal battery pressure abnormally rises because of overcharge, charge due to reverse mounting, or the like. As precautions for such a case, to prevent the batteries from bursting, safety valves for discharging gas as a cause of an internal pressure rise to the outside of the batteries are usually provided in the batteries.
However, when the safety valves do not operate normally, the batteries are likely to burst. In particular, for example, a bursting lithium ion battery expected to be mounted on an automobile could be a cause of fire. Therefore, for practical use, it is necessary to take all possible protection measures for preventing the batteries from bursting.